The present invention relates to an important improvement obtained in the manufacture of blocks of polyurethane foam, specifically the elimination, during manufacture, of the densified bottom of the blocks, obtained by the usual process.
This invention also relates to an improved apparatus for manufacturing continuous blocks of polyurethane foam and, more particularly, to arrangements for heating the housing of the conveyor belt of the foaming tunnel and parts downstream of the foaming tunnel in an apparatus of this kind.
As is known, polyurethane foam is a plastic which has acquired increasing importance over recent years, having many uses in such different fields as the automotive industry and the food industry.
In the usual process, polyurethane foam is obtained by reacting, on a moving belt, a polyol and polyisocyanate as principal ingredients, together with catalysts, dyes, blowing agents, etc. The physical properties (stiffness, density, cell size, tensile strength, etc.) of the different kinds of polyurethane foam are a function of multiple parameters, the principal ones being the nature of the polyol and of the polyisocyanate, the ratio between both and the blowing agent.
The components in liquid state are vigorously shaken immediately before being deposited on a mechanically operated endless band conveyor. Between this endless band and the poured liquids there is placed a paper web, which will follow the conveyor in its path, so that said liquids do not spill before they solidify following the reaction.
The conveyor is given translatory motion, with suitable speed for each quality of foam. The liquids deposited on the paper web commence reacting, being introduced, by the movement of the band, into a tunnel having an approximate length of 20 meters, with completion therein of the exothermic chemical reaction, via which the foam is produced, increasing by several times (approximately 50) the initial volume of the liquids. Finally there is obtained a pliable and spongy solid which is the polyurethane foam. Naturally, the shape of the cross-section of said block will be that of the tunnel at its base and side walls.
The reasons for manufacturing said polyurethane foam inside a tunnel are as follows:
(1) To prevent dispersion of the toxic gases, principally the CO.sub.2 produced in the reaction or, eventually, the toxic gases evaporated as a result of the heat produced.
(2) To obtain the approximate shape of the cross-section of the block, achieving a body with an almost rectangular section. This is usually referred to in this art as "molding effect".
Independently of the type of foam produced, a problem associated with the manufacture of the block is the densification occurring in the bottom thereof, as a result of which it is necessary to waste an important part of the finished material because its density is notably greater than that of the rest of the block.
Experts in the art will admit that it is frequent for the percentage of the densified area to be 5% by weight, even reaching 7% in relation to the total weight of the block. This amount of wasted raw material can be of considerable importance, bearing in mind that the daily production of an average-sized polyurethane foam installation exceeds 20,000 kg.
The formation of this dense area at the bottom is a serious drawback from the economical-industrial point of view because:
(1) It requires a trimming operation of said densified bottom, which consumes time, energy and manpower and requires proper mechanical means, preferably saws designed specifically for this purpose. The densified layer of foam can have a length of more than 100 meters (the length of the block), a width of two meters and a thickness of 10-30 mm.
(2) The trimmed by-product has to be marketed at a notably lower price than the rest of the foam, because of its unsatisfactory properties, its small thickness and irregular shape.
(3) Since the raw materials of polyurethane foam are substances of petrochemical origin, their high cost under present energy crisis circumstances calls for maximum utilization of the finished products which are used for manufacture.
The preceding problems are solved by the process of the invention, which provides a block without the densified area, thus attaining notable utilization of the raw materials and, consequently, a substantial saving in manufacturing costs.
As far as the inventor is aware, in this field there is no process resulting in the production of blocks without the densified bottom, as attained with the process of the invention, which can be implemented by the improved apparatus which is also an object of the invention.